Shota Kikuchi

Small bodies such as asteroids and comets present highly perturbed environments that pose significant challenges to spacecraft operations. Frozen orbits offer a practical means to maintain periodic motion; however, conventional frozen orbits are typically limited to the terminator or ecliptic planes. This paper introduces artificial frozen orbits (AFOs) around small bodies, realized by actively controlling solar radiation pressure (SRP) perturbations through solar sailing. Semi-analytical solutions for AFOs are derived using averaged Gauss variational equations incorporating dynamic SRP variations, with sail steering laws formulated as Fourier series in terms of the eccentric anomaly. The results reveal new families of AFOs that deviate from the terminator and ecliptic planes, allowing for variable orbital radii and eccentricities. A comprehensive exploration of the AFO solution space was conducted using randomly generated control profiles. Furthermore, a machine learning-based global sensitivity analysis identified that lower-order terms of the Fourier series dominate AFO formation. Finally, we present a design method to determine the optimal steering law for specified orbital elements, even when considering higher-order gravity perturbations.

Orbital and attitude motions around asteroids are strongly perturbed and coupled with each other, necessitating frequent maintenance maneuvers to ensure reliable mission operations. This study introduces a propellantless strategy for maintaining spacecraft motion by leveraging environmental disturbances, such as solar radiation pressure and asteroid gravity. The key idea is to repeatedly implement the following three operations: sun-synchronous coupled orbit-attitude motion, orbit correction via solar sailing, and momentum unloading using environmental torques. We validate this novel operation concept through both analytical and numerical analyses based on the flat-plate solar radiation pressure and ellipsoidal gravity models. The proposed method requires no specific devices except for reaction wheels, making it suitable for any asteroid mission. Such an integrated scheme contributes to reducing propellant consumption and operational workload.

The Hayabusa2 spacecraft collected samples from the surface of the carbonaceous near-Earth asteroid (162173) Ryugu and brought them to Earth. The samples were expected to contain organic molecules, which record processes that occurred in the early Solar System. We analyzed organic molecules extracted from the Ryugu surface samples. We identified a variety of molecules containing the atoms CHNOS, formed by methylation, hydration, hydroxylation, and sulfurization reactions. Amino acids, aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, and nitrogen-heterocyclic compounds were detected, which had properties consistent with an abiotic origin. These compounds likely arose from an aqueous reaction on Ryugu’s parent body and are similar to the organics in Ivuna-type meteorites. These molecules can survive on the surfaces of asteroids and be transported throughout the Solar System.

Samples of the carbonaceous asteroid (162173) Ryugu were collected and brought to Earth by the Hayabusa2 spacecraft. We investigated the macromolecular organic matter in Ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. The spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). The morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. Deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. The diversity of the organic matter indicates variable levels of aqueous alteration on Ryugu’s parent body.